Method and system for milling a fuel for an oxy-fuel combustion burner

ABSTRACT

The present disclosure relates to a method of milling a fuel for an oxy-fuel combustion burner, the method includes: separating air into a hot nitrogen gas stream, having a temperature of at least 150° C. and a purity of at least 98 mol-% nitrogen, and an oxygen gas stream; leading at least a part of the nitrogen gas stream to a fuel mill; milling the fuel by means of the fuel mill in a nitrogen rich atmosphere formed by means of the nitrogen gas stream; leading the at least a part of the nitrogen gas stream away from the milled fuel; leading the oxygen gas stream to the oxy-fuel combustion burner; conveying the milled fuel to the oxy-fuel combustion burner; and burning the fuel, by means of the oxy-fuel combustion burner, in an oxygen rich atmosphere formed by means of the oxygen gas stream. The present disclosure further relates to a system for milling a fuel for an oxy-fuel combustion burner as well as to a power plant comprising such a system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to PCT/IB2012/000232 filed Feb. 8,2012, which in turn claims priority to European Application 11154391.4filed Feb. 14, 2011, the contents of which are each incorporated intheir entirety.

TECHNICAL FIELD

The present disclosure relates to a method and a system for milling afuel for an oxy-fuel combustion burner. The present disclosure alsorelates to an oxy-fuel combustion power plant comprising such a system.

BACKGROUND

Most of the energy used in the world today is derived from thecombustion of carbon and hydrogen containing fuels such as coal, oil andnatural gas, as well as other organic fuels. Such combustion generatesflue gases containing high levels of carbon dioxide. Due to the concernsabout global warming, there is an increasing demand for the reduction ofemissions of carbon dioxide to the atmosphere, why methods have beendeveloped to remove the carbon dioxide from flue gases before the gas isreleased to the atmosphere.

In order to reduce the amount of flue gas, and thus the dimensions of apower plant and its gas cleaning arrangements, as well as to facilitatethe purification and removal of carbon dioxide, oxygen may be usedinstead of air in a combustion furnace, so called oxy-fuel combustion,generating a flue gas with a high carbon dioxide concentration and a lownitrogen concentration. The oxygen may be obtained from separating airinto an oxygen stream and a nitrogen stream by means of an airseparation unit (ASU).

The fuel for oxy-fuel combustion, such as coal, may be milled to apowder prior to entry into the furnace in order to improve thecombustion.

JP 59-024115 A2 discloses separation of air into nitrogen enriched airand oxygen enriched air by means of an oxygen permeable membrane, afterwhich coal is pulverised under the nitrogen enriched air and thepulverised coal is burned with the oxygen enriched air.

SUMMARY

According to an aspect of the present disclosure, there is provided amethod of milling a fuel for an oxy-fuel combustion burner, the methodcomprising: separating air into a hot nitrogen gas stream, having atemperature of at least 150° C. and a purity of at least 98 mol-%nitrogen, and an oxygen gas stream; leading at least a part of thenitrogen gas stream to a fuel mill; milling the fuel by means of thefuel mill in a nitrogen rich atmosphere formed by means of the nitrogengas stream; leading the at least a part of the nitrogen gas stream awayfrom the milled fuel; leading the oxygen gas stream to the oxy-fuelcombustion burner; conveying the milled fuel to the oxy-fuel combustionburner; and burning the fuel, by means of the oxy-fuel combustionburner, in an oxygen rich atmosphere formed by means of the oxygen gasstream.

According to an other aspect of the present disclosure, there isprovided a system comprising: an air separation unit (ASU) arranged forseparating air into a hot nitrogen gas stream having a temperature of atleast 150° C. and a purity of at least 98 mol-% nitrogen, and an oxygengas stream; a fuel mill, arranged for milling a fuel in a nitrogen richatmosphere formed by means of the nitrogen gas stream; and an oxy-fuelcombustion burner, arranged for burning the milled fuel in an oxygenrich atmosphere formed by means of the oxygen gas stream.

According to an other aspect of the present disclosure, there isprovided an oxy-fuel combustion power plant comprising the system of theabove aspect.

Discussions above and below relating to any one of the respectiveaspects of the present disclosure are also, in applicable parts,relevant to any of the other aspects.

By obtaining a hot nitrogen gas stream having a high temperature andleading it to the fuel mill, the fuel may be milled under an atmosphereof the hot nitrogen gas stream. Thus, the hot gas stream may act to drythe fuel, while also providing an inert environment which reduces therisk of dust explosions during milling/pulverizing the fuel. It may bedesirable to dry the fuel prior to burning it in order to increase theenergy efficiency of the burner and reduce the amount of water vapourcontaminating and increasing the volume of the flue gas. Before reachingthe mill, the nitrogen gas stream may have been expanded and itstemperature may have been reduced, but may still be high enough to atleast partially dry the fuel in the mill. By obtaining a nitrogen gasstream from the ASU with a high enough temperature, it may not benecessary to heat the nitrogen gas stream by means of an additional heatexchanger, externally of the ASU, before said gas stream may be used fordrying the fuel.

At least a major part of the nitrogen gas stream is removed from thefuel before the fuel reaches the burner, in order to reduce the amountof nitrogen in the flue gas, thereby reducing the flue gas volume.

BRIEF DESCRIPTION OF THE DRAWINGS

Currently preferred embodiments will below be discussed with referenceto the drawings, in which:

FIG. 1 is a schematic diagram of an embodiment of a power plantcomprising a system in accordance with the present disclosure.

FIG. 2 is a schematic diagram of an embodiment of an air separation unitthat may be comprised in a system in accordance with the presentdisclosure.

FIG. 3 is a schematic process chart of an embodiment of a method inaccordance with the present disclosure.

DETAILED DESCRIPTION

The system and power plant of the present disclosure comprises pipingthat connects their different parts and is arranged to allow respectivefluids and solid materials to flow or be transported through thesystem/power plant as needed. The piping may comprise conduits, valves,pumps, conveyors, compressors, fans, expanders, nozzles, heat exchangersetc. as appropriate to control the flow/transportation and properties ofrespective fluids and solids.

The hot nitrogen gas stream has a temperature of at least 100° C., suchas at least 150° C., at least 200° C., at least 220° C. or at least 240°C., e.g. between 200° C. and 300° C., between 220° C. and 280° C. orbetween 240° C. and 260° C., or about 250° C. This high temperature maybe obtained between the separation of the nitrogen stream from the airand of said nitrogen stream entering the fuel mill, such as when leavingthe ASU after heat exchanging with compressed air within the ASU, butthe temperature might not be so high all the way to the fuel mill. Thehot nitrogen gas stream may e.g. have been cooled somewhat by expansion,e.g. in a turbine for energy recovery, before reaching the fuel mill.Conveniently, the nitrogen gas stream may still be of elevatedtemperature when reaching the fuel mill such that it may act to dry thefuel within the mill. Thus, the nitrogen gas stream, when entering thefuel mill, may have a temperature of at least 100° C., at least 120° C.or least 140° C., e.g. between 100° C. and 200° C., between 120° C. and180° C. or between 140° C. and 160° C., or about 150° C.

The hot nitrogen gas stream has a high purity of at least 95 mol-%nitrogen, such as at least 98 mol-%, at least 99 mol-% or at least 99.5mol-%. This high purity of the nitrogen gas stream increases itsinertness and reduces the risk of ignition or dust explosion, or thelike, of the fuel during milling, drying and/or transportation of thefuel before or after milling.

It may also be convenient with a high purity of the oxygen gas stream inorder to improve the energy efficiency of the oxy-fuel combustion burnerand reduce the flue gas volume. Thus, the oxygen gas stream may have ahigh purity of at least 90 mol-% oxygen, such as at least 95 mol-%, atleast 98 mol-%, at least 99 mol-% or at least 99.5 mol-%.

The separating of air into a hot nitrogen gas stream and an oxygen gasstream may be performed in any suitable way, such as by comprisingcryogenic distillation of the air. Cryogenic distillation may beconvenient for the disclosed method since nitrogen and oxygen stream maybe obtained with very high purity, such as of at least 95 mol-%, atleast 98 mol-% or at least 99 mol-% nitrogen and oxygen, respectively.

The separating of air into a hot nitrogen gas stream and an oxygen gasstream may comprise compression of the air prior to separation.Conveniently, the compression might be at least partly adiabatic toobtain compressed air of a high temperature, such as of at least 150°C., at least 180° C., at least 200° C., at least 250° C. or at least300° C., e.g. of between 250° C. and 350° C., between 280° C. and 320°C. or about 300° C. This hot compressed air may then be used to obtainthe high temperature of the hot nitrogen gas stream by heat exchange.Thus, the compressed air may be chilled before separation, and theseparated nitrogen stream may be heated. Compression may be performed ina single compression step, or by multistage compression.

Typically, multistage compression with intermittent cooling is used forcompressing the air in an ASU. In most cases the heat is then removed byusing cooling water. The removed heat may then leave the plant at about40° C. and will end up in the cooling tower, air cooler etc. In thatcase, the heat will be lost since it is not further utilized in thesystem or power plant. An advantage of intercooled compression is thelower required shaft power. It is thus surprising that a lower overallenergy consumption may be obtained by using adiabatic compression,allowing heating of the nitrogen gas stream to a temperature such thatit may be used for drying the fuel with less or no additional heating ofthe nitrogen gas stream and less or no additional drying of the fuel. Inaddition, the oxygen required for fuel combustion in the boiler may bepreheated. This reduces the required demand of heating media, such ashot flue gas from fuel combustion, steam from water-steam-cycle, or hotboiler feed water from water-steam-cycle. Thus the overall energyconsumption (fuel consumption) may be reduced. Also, using adiabatic aircompression with waste heat utilization may save cooling water. In caseof burning hard coal or sub bituminous coal the reduction of fuelmoisture allows for reduction of the size of all flue gas ducts and mayalso reduce the amount of cooling water required for condensing andremoving the water vapour of the flue gas.

Thus the overall heat balance as well the water balance of an Oxy-powerplant is improved.

It should be noted that intercooled compression, or a combination ofadiabatic and intercooled compression, may also be used with the methodand system of the present disclosure as long as a nitrogen gas stream ofsufficiently high temperature may be obtained. The compression may beconfigured such that the overall power consumption of the system orpower plant is reduced or minimised.

All or only a part of the nitrogen gas stream produced by the ASU may beled to the fuel mill. If only a part is led to the mill, a second partmay be led to an additional fuel dryer, complementing the dryingperformed in the fuel mill. Using an additional dryer may be especiallyconvenient in case of burning fuels with a high moisture content (e.g.lignite). Devices and methods for drying fuels with high water contentsuch as lignite are well known for conventional power plants. The heatdemand of such conventional dryers as well the dimensions of the samemay thus be reduced by integrating the dryer with the ASU (waste) heatin accordance with the present disclosure. The additional dryer may bearranged to dry the fuel before or after it is milled, but it may beconvenient to additionally dry the fuel after milling since milled fuelmay have a larger contact surface with the hot nitrogen gas stream thannon-milled fuel, facilitating drying. Thus, the method of the presentdisclosure may also comprise: leading a second part of the hot nitrogengas stream to a fuel dryer; drying the fuel by means of the fuel dryer,before or after the milling of said fuel; and leading the second part ofthe hot nitrogen gas stream away from the dried fuel. Similarly, one ormore additional dryers may be used consecutively as needed, either usingparts of the nitrogen gas stream or other drying medium for drying thefuel.

The nitrogen rich atmosphere of the fuel mill, and/or of any additionaldryer, is formed by means of the nitrogen rich gas stream. However, thenitrogen rich gas stream might not only be formed by said nitrogen gasstream. It may be convenient to recycle flue gas from downstream of theburner to the mill to help form a suitable inert and drying atmospherefor the mill. By combining the nitrogen gas stream with recycled fluegas to form the mill, and/or additional dryer, atmosphere, heat from theburner and fuel combustion may be additionally used for drying the fueland less of the hot nitrogen gas stream may be needed.

Similarly, the oxygen rich atmosphere at the burner may be formed by notonly the oxygen gas stream from the ASU, but also by recycled flue gas.

Any suitable conduit, such as a pipe, possibly in cooperation with afan, a compressor, an expander such as a turbine and/or other unit asconvenient, may be used for leading the nitrogen gas stream from the ASUto the fuel mill, leading the nitrogen gas stream away from the fuel,e.g. away from the fuel mill or away from a conveyor transporting thefuel from the mill towards the burner, and leading the oxygen gas streamfrom the ASU to the burner. If a second part of the nitrogen gas streamis used for drying the fuel in an additional dryer, in addition to themill, such a suitable conduit may also be used for leading said secondpart of the nitrogen gas stream from the ASU to the fuel dryer and forleading the second part of the nitrogen gas stream away from the fuel,e.g. away from the fuel dryer or away from a conveyor transporting thefuel from the dryer towards the burner.

Any suitable conveyor, such as a conveyor belt may be used to convey ortransport the unmilled fuel to the fuel mill and/or the milled fuel fromthe fuel mill to the burner, possibly via an, or several, additionalfuel dryer(s).

The system of the present disclosure may thus also comprise: a firstconduit arranged for leading at least a part of the nitrogen gas streamto the fuel mill; a second conduit arranged for leading the at least apart of the nitrogen gas stream away from the milled fuel; a thirdconduit arranged for leading the oxygen gas stream to the oxy-fuelcombustion burner; and a conveyor arranged for conveying the milled fuelto the oxy-fuel combustion burner.

The mill may be any suitable fuel mill for milling/pulverising the fueland allowing a nitrogen gas stream to pass through it.

The additional dryer, if used, may be any suitable fuel dryer for dryingthe fuel and allowing a nitrogen gas stream to pass through it.

The burner may be any suitable oxy-fuel combustion burner.

In order to avoid fuel particles, e.g. milled fuel powder particles,following the nitrogen gas stream away from the fuel bulk during millingand drying of the fuel by means of the nitrogen gas stream, the conduitarranged to lead the nitrogen gas stream away from the fuel may bearranged with, or otherwise in cooperation with, a particle remover suchas a an electrostatic precipitator, a cyclone, a filter and/or ascrubber. If an additional dryer is used, a particle remover may be incooperation with the conduit arranged to lead the nitrogen gas streamaway from the additionally dried fuel. Alternatively, a single particleremover may be used for both the nitrogen gas stream from the mill andthe nitrogen gas stream from the additional dryer.

With reference to FIG. 1, a specific power plant 1 comprising a systemin accordance with the present disclosure will now be described.

The power plant 1, as well as the system, comprises a fuel mill 2arranged for milling/pulverising the power plant fuel, such as coal. Themill 2 is connected via a conduit or conveyer for entering fuel to bemilled into the mill 2. The mill 2 is also connected via a conduit 3 tothe air separation unit (ASU) 4 for allowing a nitrogen gas stream fromthe ASU 4 to enter the mill 2 via a gas inlet of the mill 2. Further,the mill 2 is arranged to receive recycled flue gas via a conduit and agas inlet of the mill 2, the flue gas being recycled by means of a fan,compressor or turbine 5. A conduit 6 is arranged to connect the mill 2and a particle remover in the form of an electrostatic precipitator(ESP) 7 such that the nitrogen gas stream and/or the recycled flue gasmay exit the mill 2 via a gas outlet of the mill 2 and enter the ESP 7via a gas inlet of the ESP 7. The ESP 7 is arranged to remove and returnany fuel particles that has followed the nitrogen gas stream. A conveyorin the form of a conveyor belt 8 connects the mill 2 with an oxy-fuelcombustion burner 9 such that milled fuel may be transported by means ofthe conveyor 8 from the mill 2 to the burner 9. The gas inlets andoutlet of the mill 2 are arranged such that the nitrogen and flue gasgas streams passes through the mill 2 transversely to the direction inwhich the fuel is transported by means of the conveyor 8, thus reducingthe amount of flue gas and, especially, nitrogen gas that reaches theburner 9. Thus the mill 2 may mill the fuel in an inert nitrogen andflue gas atmosphere that reduces the risk of fire or explosion in themill 2, while the streaming hot gas also dries the fuel.

An additional fuel dryer 10 may also be used. The dryer 10 is similarlyconnected to the ASU 4 via a conduit 11 and to the ESP 7 via a conduit12, allowing hot nitrogen gas to stream into the dryer 10 via a gasinlet, pass through the milled fuel transported by the conveyor 8 whiledrying said fuel and in a direction perpendicular of the transportdirection of the fuel, and stream out of the dryer 10 via a gas outletand the conduit 12 towards the ESP 7.

Additionally or alternatively to the dryer 10, a fuel dryer 13 using adrying medium other than the hot nitrogen gas stream may be used fordrying the fuel.

The mill 2, as well as the conveyor 8 and dryers 10 and 13 may beoperated at a pressure slightly above ambient in order to avoidin-leakage of air, and thus nitrogen, which would lessen the advantageswith oxy-fuel combustion.

The oxy-fuel combustion burner 9 is arranged in or with the boiler 14and is arranged to receive the dried and milled fuel via the conveyor 8.The burner 9 is configured to combust the fuel in an oxygen richatmosphere provided by means of an oxygen rich gas stream from the ASU 4arranged to enter the boiler 14 via a conduit connecting the boiler 14with the ASU 4 and via a gas inlet of the boiler 14, possibly togetherwith flue gas recycled by means of a fan, compressor or turbine 15. Byallowing the burner 9 to operate with oxygen instead of air, the amountof flue gas is reduced since the inert nitrogen has been previouslyremoved. The burning may be controlled by ratio of recycled flue gas tooxygen used. In the event of a failure of the ASU or if oxygen canotherwise not be provided to the burner 9, combustion with air may stillbe possible to ensure reliability of the plant 1.

The boiler 14 is arranged to produce steam from the heat produced by theburner 9, the steam being used for generation of electricity by means ofturbines (not shown).

The ASU 4 is, as mentioned above, connected to the mill 2, the dryer 10and the boiler 14 via gas conduits such that the hot nitrogen gas streamproduced by the ASU 4 may pass through the mill 2 by means of conduits 3and 6 and through the dryer 10 via conduits 11 and 12, and the oxygengas stream produced by the ASU may enter the boiler 14 to be used forburning the fuel at the burner 9. If desired, the hot nitrogen gasstream from the ASU may be additionally heated by means of a heatexchanger 16 before entering the mill 2 and the dryer 10, respectively.It may be convenient to pre-heat the oxygen gas stream by means of aheat exchanger 17 before the stream enters the boiler 14.

A flue gas cleaning arrangement is connected to the boiler 14 to cleanthe flue gas produced by the oxy-fuel combustion. The flue gas may thusconsecutively pass through several different cleaning units before anyvent gas is released to the atmosphere. In the specific power plant ofFIG. 1, the flue gas passes through a flue gas heat exchanger arrangedto cool the flue gas exiting the boiler 14 and to heat the flue gasrecycled to the mill 2 and the boiler 14 as discussed above, anelectrostatic precipitator 19 for removing particles from the flue gas,a flue gas compressor 20, a flue gas cooler 21, a wet flue gasdesulphurisation unit 22, a flue gas condenser 23, and a gas processingunit 24 for removing carbon dioxide from the flue gas.

With reference to FIG. 2, a specific embodiment of an ASU 4 will now bedescribed.

An adiabatic compressor 25 is configured to compress air of ambienttemperature and pressure to a temperature of between 200° C. and 300°C., such as 200° C. and 250° C. and a pressure of between 2-20 bar, suchas 3-6 bar, e.g. about 5 bar. The compressor is arranged such thatincoming air via a conduit may enter the compressor 25 via a gas inlet.The compressor 25 is connected to a heat exchanger 26 via a gas conduitsuch that compressed air may exit the compressor 25 via a gas outlet ofthe compressor 25 and the conduit to enter the heat exchanger 26 via gasinlet of the heat exchanger 26 to be chilled by said heat exchanger 26.

A cryogenic distillation unit 27 is in fluid connection with the heatexchanger 26 such that chilled compressed air, which may be at leastpartially liquefied, may enter the distillation unit 27 via a fluidconduit and a fluid inlet of the distillation unit 27. The distillationunit 27 may e.g. be a conventional distillation unit used inconventional cryogenic ASUs. The distillation unit 27 is arranged tocryogenically distil the compressed air such that the air is separatedinto at least one nitrogen fluid stream, which may be gas or liquid or amixture thereof, with a purity of 99.5 mol-% and at least one oxygenfluid stream, which may be gas or liquid or a mixture thereof, andpossibly an argon fluid stream and/or streams of other air constituents.Further, the distillation unit 27 is in fluid connection with the heatexchanger 26 such that the nitrogen and oxygen fluid streams may exitthe distillation unit 27 via respective fluid outlets and pass into theheat exchanger 26 via respective conduits and fluid inlets of the heatexchanger 26.

An expander in the form of a turbine 28 is arranged to expand the atleast one nitrogen stream and possibly also other of the separationproducts, while recovering energy from said expansion. The turbine 28 isin fluid connection with the heat exchanger 26 via conduits for thenitrogen stream and optionally the oxygen gas stream, respectively, suchthat the nitrogen and oxygen gas streams may exit the heat exchanger 26via respective gas outlets of the heat exchanger 26, pass via respectiveconduits from the heat exchanger 26 to the turbine 28 and enter theturbine 28 via respective gas inlets. Typically, the expander 28comprises separate turbines for the nitrogen and oxygen streamsrespectively. The expander 28 may be arranged for multistage expansionThe expander 28 is arranged to expand the hot nitrogen gas stream from atemperature of about 250° C. and a pressure of between 2-20 bar, such as3-6 bar, e.g. about 5 bar, to a temperature of about 150° C. and apressure which is only slightly above ambient. After expansion by meansof the expander 28, the oxygen gas stream may be directed to the boiler14 and the nitrogen gas stream may be directed to the mill 2 andpossibly the dryer 10, as discussed above in respect of FIG. 1.

As is shown in FIG. 2, at least a part of the nitrogen gas stream fromthe distiller 27 may additionally or alternatively bypass the heatexchanger 26 and/the expander 28, e.g. to be directly led to the mill 2and/or the dryer 10, possibly via the optional pre-heating heatexchanger 16 of FIG. 1.

It may be convenient to also expand the oxygen stream in an expander,e.g. expander 28, for instance to recover energy. The oxygen stream maythereafter be directed to the boiler 14 and the burner 9. It may beconvenient, especially if the oxygen stream has been expanded, topre-heat the oxygen stream before entering the boiler 14, e.g. by meansof heat exchange with steam. As is shown in FIG. 2, the oxygen streammay either be expanded in the expander 28 or bypass the expander 28,e.g. being led directly to the boiler 14, or part of the oxygen streammay pass through the expander 28 while another part bypasses expander28.

Optionally, a heat exchanger 29 may be used to pre-heat the nitrogen gasstream, and optionally the oxygen gas stream, prior to expansion bymeans of the expander 28. The heat exchanger 29 may as heating mediume.g. use hot flue gas from the boiler 14, steam from the water/steamcycle of the power plant 1 and/or boiler feed water from the water/steamcycle of the power plant 1.

The heat exchanger 26 is in fluid connection with the compressor 25, thedistillation unit 27 and the expander 28, as discussed above. The heatexchanger 26 is arranged to cool the compressed air by means of heatexchanging with the distillation products, i.e. the nitrogen and oxygengas streams. An additional cooling medium in addition to the nitrogenand oxygen gas streams may also be needed. The heat exchanger 26 maythus be arranged to cool the compressed air from a temperature ofbetween 200° C-250° C. and a pressure of between 2-20 bar, such as 3-6bar, e.g. about 5 bar, to a temperature of between 50° C. to 100° C. anda pressure of between 2-20 bar, such as 3-6 bar, e.g. about 5 bar, toheat the nitrogen gas stream from a temperature of between 0° C. to 30°C., such as about 10° C. and a pressure of between 2-20 bar, such as 3-6bar, e.g. about 5 bar, to a temperature of between 150° C-250° C. and apressure of between 2-20 bar, such as 3-6 bar, e.g. about 5 bar, and toheat the oxygen gas stream from a temperature of between 0° C.-30° C.,such as about 10° C., and a pressure of between 1-20 bar, such as 1-3bar, e.g. about 1.2 bar to a temperature of between 150° C-250° C. and apressure of between 1-3 bar, e.g. about 1.2 bar.

With reference to FIG. 3, a specific embodiment 100 of a method inaccordance with the present disclosure will now be described.

By means of the ASU 4, air is separated, step 101, into a hot nitrogengas stream and an oxygen gas stream. The hot nitrogen gas stream is ledto the fuel mill 2 and the oxygen gas stream is led to the oxy-fuelcombustion burner 9.

In the fuel mill 2, fuel such as coal is milled, step 102, under anitrogen rich atmosphere formed by means of the hot nitrogen gas streamfrom the ASU 4. The nitrogen gas stream is allowed to flow through themill 2 such that at least most of the nitrogen gas is removed, step 103,from the fuel and does not follow the fuel to the burner 9.

The milled fuel is transported by means of the conveyor 8 to the burner9 where it is oxy-fuel combusted, step 104, under an oxygen richatmosphere formed by means of the oxygen gas stream from the ASU 4.

While the invention has been described with reference to a number ofpreferred embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiments disclosed as the best modecurrently contemplated for carrying out this invention, but that theinvention will include all embodiments falling within the scope of theappended claims. Moreover, the use of the terms first, second, etc. donot denote any order or importance or chronology, but rather the termsfirst, second, etc. are used to distinguish one element from another.

1. A method of milling a fuel for an oxy-fuel combustion burner, themethod comprising: separating air into a hot nitrogen gas stream, havinga temperature of at least 150° C. and a purity of at least 98 mol-%nitrogen, and an oxygen gas stream; leading at least a part of thenitrogen gas stream to a fuel mill; milling the fuel by means of thefuel mill in a nitrogen rich atmosphere formed by means of the nitrogengas stream; leading the at least a part of the nitrogen gas stream awayfrom the milled fuel; leading the oxygen gas stream to the oxy-fuelcombustion burner; conveying the milled fuel to the oxy-fuel combustionburner; and burning the fuel, by means of the oxy-fuel combustionburner, in an oxygen rich atmosphere formed by means of the oxygen gasstream.
 2. The method of claim 1, wherein the air separating comprisescryogenic distillation.
 3. The method of claim 1, wherein the airseparating comprises adiabatic compression of air to form an air streamhaving a temperature of at least 150° C.
 4. The method of claim 1,wherein the leading away of the hot nitrogen gas stream comprisesremoving any fuel particles from the hot nitrogen gas stream by means ofa particle remover, such as an electrostatic precipitator or a cyclone.5. The method of claim 1, further comprising: leading a second part ofthe hot nitrogen gas stream to a fuel dryer; drying the fuel by means ofthe fuel dryer, before or after the milling of said fuel; and leadingthe second part of the hot nitrogen gas stream away from the dried fuel.6. A system comprising: an air separation unit arranged for separatingair into a hot nitrogen gas stream having a temperature of at least 150°C. and a purity of at least 98 mol-% nitrogen, and an oxygen gas stream;a fuel mill, arranged for milling a fuel in a nitrogen rich atmosphereformed by means of the nitrogen gas stream; and an oxy-fuel combustionburner, arranged for burning the milled fuel in an oxygen richatmosphere formed by means of the oxygen gas stream.
 7. The system ofclaim 6, wherein the air separation unit comprises a cryogenicdistillation unit.
 8. The system of claim 6, wherein the air separationunit comprises an adiabatic compressor arranged for compressing air toform an air stream having a temperature of at least 150° C.
 9. Anoxy-fuel combustion power plant comprising the system of claim 6.